In order to improve the ride quality of a vehicle, such as a road vehicle, it is well known that the vehicle body including the passenger carrying compartment should be isolated from the road wheels by a suspension system.
In known vehicles, it is conventional that the vehicle is provided with a number of road wheels. Typically four are provided on a passenger vehicle—although more may be provided on vehicles such as coaches or heavy goods vehicles. Each wheel supports a rubber road tyre which engages the road surface to form a “contact patch” at the circumference of the tyre. The wheel is supported by a wheel carrier and rotates around an axis passing through the wheel carrier which is perpendicular to the plane of the wheel and the direction of rotation of the wheel.
For the sake of clarity of description, the road wheel in its normal position of use upon a road surface is considered to lie in a vertical plane, with the axis of rotation of the wheel being horizontal and orthogonal to the direction of travel of the vehicle as the wheel rotates when the vehicle is travelling in a straight line longitudinal to the vehicle.
Each wheel carrier is operatively connected to the vehicle body by the suspension system. In general, any suspension system can be considered to comprise one or more links making up a linkage which controls movement of the wheel carrier relative to the body. The links may connect the wheel carrier directly to the vehicle body or to a subframe which is in turn attached to the vehicle body. Generally, the linkage is the portion of the suspension system that provides the majority of the travel of the wheel carrier relative to the body. The subframe may also allow a much more limited range of movement.
In order to provide a smooth ride for occupants of the vehicle and good body control during acceleration, braking and cornering, the suspension must fulfil several roles.
The first role of the suspension system is to isolate the vehicle body from road inputs. To perform successfully, this typically requires a “soft” suspension. However, the second role is to control the wheel relative to the body during braking, acceleration and cornering forces which demands a “hard” suspension for good control and rapid responses. Obviously, these two roles are conflicting when considered in such a broad brush manner. Designers have in the past believed it necessary to produce a compromise design depending on the intended function of the vehicle, i.e. sports or luxury vehicle.
One specific area of compromise, and which is the basis of the invention, relates to this stiffness of the suspension system, and specifically the deflection of the wheel (and more specifically the wheel carrier) under operating forces as controlled by the suspension system.
As set out hereinbefore, the suspension system can be thought of as the complete set of components linking the vehicle wheel to the vehicle body. This may include a subframe and a suspension sub-set comprising a number of links defining a linkage which allow the vast majority of wheel travel (mostly vertical displacement) to be achieved.
In a typical system the wheel carrier is attached to the suspension system using a wheel bearing that constrains all degrees of freedom of the wheel with respect to the suspension system with the exception of rotation about the bearing axis. In force terms, this means that the wheel can only impart a force on the suspension system that is either parallel to or passes through the bearing axis.
During motion of the vehicle in a straight line along an imperfect road, impact forces are created which are typically in the plane of the wheel and consist of varying combinations of longitudinal, lateral and vertical forces. These are applied to the suspension at the centre of the wheel due to the wheel bearing. In the main, reaction of the vertical forces is provided by a flexible linkage consisting of a spring/damper combination, whilst reaction of the longitudinal and lateral forces is provided by the suspension linkage which can be a rigid link since only limited travel is needed, or could be a flexible link.
It is now widely accepted that the lower the longitudinal stiffness of the suspension linkage (i.e. freedom to move backwards and forwards along direction of travel of vehicle), the lower the transmission of impact forces into the vehicle and hence the more refined the vehicle is.
As with the broad view of suspension design considered hereinbefore, selecting the longitudinal stiffness of the suspension system has, in the past, been thought to be a comprise between good road handling and comfort. The reason for this belief is that both braking and impact forces produce longitudinal displacement of the wheel carrier. Under braking, the location of the wheel carrier by the suspension is required to be longitudinally stiff to prevent loss of castor angle, yet for good impact absorption it should have a low stiffness longitudinally.
The applicants have appreciated that whilst both braking and impact absorption place broadly conflicting requirements on vehicle suspension longitudinal stiffness, the manner in which the displacement of the wheel carrier is produced due to braking and impact loads is different.
Under braking, with an outboard braking system acting between a wheel carrier and wheel 2, the braking system provides a torque reaction between the wheel and suspension links which the bearing normally prevents. This results in longitudinal forces BF acting at the location of the tyre contact patch 202 (in the same way as if a longitudinal force were applied to the tyre at the contact patch with the brake locked). However, for impact forces IF although a longitudinal displacement also occurs it is due to forces that act through the centre of the wheel carrier rather than of the contact patch. This can be seen in FIG. 1 of the accompanying drawings. The height of the vehicle also generates vertical forces VF acting through the wheel carrier.
The applicants thus propose to provide a suspension system that provides good performance under various longitudinal forces by taking into account the different nature of those longitudinal forces which act through either the contact patch or wheel centre. Such an approach to suspension system design has heretofore not been fully exploited by the skilled person in the art.
Suspension systems have been produced in which the contact patch longitudinal stiffness is greater than the wheel carrier longitudinal stiffness by suitable arrangement of the mounting for the vehicle subframe. An example is the suspension for the McLaren F1 motor car which employs a set of four subframe mounting bushes that are radially stiff and are focused on the wheel contact patch. A problem with such an approach is that the entire mass of the subframe must translate longitudinally during a deflection of an impact.